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How to Teach and Think About Spontaneous Wave Function Collapse Theories: Not Like Before

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How to Teach and Think About Spontaneous Wave Function Collapse Theories: Not Like Before How to teach and think about spontaneous wave function collapse theories: not like before Lajos Di´osi Wigner Research Center for Physics H-1525 Budapest 114. P.O.Box 49, Hungary (Dated: October 28, 2017) A simple and natural introduction to the concept and formalism of spontaneous wave function collapse can and should be based on textbook knowledge of standard quantum state collapse and monitoring. This approach explains the origin of noise driving the paradigmatic stochastic Schr¨odinger equations of spontaneous localiza- tion of the wave function Ψ. It reveals, on the other hand, that these equations are empirically redundant and the master equations of the noise-averaged stateρ ^ are the only empirically testable dynamics in current spontaneous collapse theories. I. INTRODUCTION \We are being captured in the old castle of standard quantum mechanics. Sometimes we think that we have walked into a new wing. It belongs to the old one, however" [1]. Year 1986 marked the birth of two theories, prototypes of what we call theory of spon- taneous wave function collapse. Both the GRW paper published in Physical Review D [3] followed by Bell's insightful work [4] and the author's Thesis [5] constructed strict stochastic jump equations to explain unconditional emergence of classical behavior in large quantum systems. Subsequently, both theories obtained their time-continuous versions, driven by white-noise rather than by stochastic jumps. The corresponding refinement of the GRW pro- posal [6{8] is the Continuous Spontaneous Localization (CSL) theory, the author's gravity- related spontaneous collapse theory [5, 9, 10] used to be called DP theory after Penrose concluded to the same equation for the characteristic time of spontaneous collapse in large bodies [11]. These theories modified the standard theory of quantum mechanics in order to describe the irreversible process of wave function collapse. The mathematical structure of modification surprised the proponents themselves and it looked strange and original for 2 many of the interested as well. In fact, these theories were considered new physics with new mathematical structures, to replace standard equations like Schr¨odinger's. The predicted effects of spontaneous collapses are extreme small and have thus remained untestable for the lack of experimental technique. After three decades, fortunately, tests on nanomasses are now becoming gradually available. Theories like GRW, CSL, DP have not changed over the decades apart from their parameter ambiguities, see reviews by Bassi et al. [12, 13]. But our understanding and teaching spontaneous collapse should be revised radically. Personally, I knew that GRW's random jumps looked like unsharp measurements but, in the late nineteen-eighty's, I believed that unsharp measurements were phenomenologi- cal modifications of von Neumann standard ones. My belief extended also for the time- continuous limit of unsharp measurements [14] that DP collapse equations [10] were based on. Finally in the nineteen-nineties I got rid of my ignorance and learned that unsharp measurements and my time-continuous measurement (monitoring) could have equally been derived from standard quantum theory [15, 16]. That was disappointing [1]. Excitement about the radical novelty of our modified quan- tum mechanics evaporated. Novelty got reduced to the concept that tiny collapses, that get amplified for bulk degrees of freedom, happen everywhere and without measurement devices. That's why we call them spontaneous. But they are standard collapses otherwise. I have accordingly stressed upon their revised interpretation recently [2], the present work is arguing further toward such demand. II. HOW TO TEACH GRW SPONTANEOUS COLLAPSE? We should build as much as possible on standard knowledge, using standard concepts, equations, terminology. Key notion is unsharp generalized measurement, which has been standard ever since von Neumann showed how inserting an ancilla between object and mea- suring device will control measurement unsharpness [17]. Hence we are in the best pedagog- ical position to explain GRW theory to educated physicists. No doubt, for old generations measurement means the projective (sharp) one but this has changed recently due to the boom in quantum information science. For younger scientists, generalized measurements are the standard ones, projective measurements are the specific case [18, 19]. For new gen- eration, there is a natural way to get acquainted with spontaneous collapse. The correct 3 and efficient teaching goes like this. GRW theory assumes that independent position measurements of unsharpness (precision) p −5 rC = 2, with GRW choice rC = 10 cm, are happening randomly at average frequency −19 λ = 10 Hz on each (non-relativistic) particle in the Universe. The two parameters rC ; λ are considered new universal constants of Nature. The mathematical model of unsharp measurements is exactly the same as for independent von Neumann detectors [17] where p the Gaussian ancilla wave function has the width σ = rC = 2 [1]. The difference from standard von Neumann detection is the concept of being spontaneous: GRW measurements are supposed to happen without presence of detectors. The merit of GRW is wave function localization in bulk degrees of freedom like, e.g., the center-of-mass (c.o.m.) of large objects. Quantum theory allows for arbitrary large quantum fluctuations of macroscopic degrees of freedom in large quantized systems. The extreme ex- ample is a Schr¨odingercat state where two macroscopically different wave functions would be superposed. In GRW theory such macroscopic superpositions or fluctuations become suppressed by GRW spontaneous measurements but the superpositions of microscopic de- grees of freedom will invariably survive. These complementary features are guaranteed by the chosen values of parameters σ and λ. Due to the extreme low rate of measurements, individual particles are almost never measured. But among an Avogadro number (A) of constituents some N = Aλ ∼ 104 become spontaneously measured in each second, meaning that their collective variables, e.g.: center-of-mass, become measured each second at preci- p sion σ= N ∼ 10−7 cm, leading to extreme sharp c.o.m. localization on the long run. That's what we expect of spontaneous localization theories. The mathematical model is the following. We model the Universe or part of it by a quantized N-body system satisfying the Schr¨odingerequation d jΨi i = − H^ jΨi (1) dt ~ apart from instances of spontaneous position measurements that happen randomly and inde- pendently at rate λ on every constituent. Spontaneous position measurements are standard generalized measurements. Accordingly, when the k'th coordinate x^k endures a measure- ment, the quantum state undergoes the following collapse: p G(xk − x^k) jΨi jΨi =) p : (2) k G(xk − x^k) jΨi k 4 The effects of unsharp position measurement take the Gaussian form: 1 (x − x^)2 G(x − x^ ) = exp − ; (3) k k (2πσ2)3=2 2σ2 where xk is the random outcome of the unsharp position measurement on x^k, and σ sets the scale of unsharpness (precision). The probability of the outcomes xk is defined by the standard rule: p 2 p(xk) = k G(xk − x^k) jΨi k : (4) We have thus specified the mathematical model of GRW in terms of standard unsharp position measurements targeting every constituent at rate λ and precision σ. These mea- surements are selective measurements if we assume that the measurement outcomes xk are accessible. If they are not, we talk about non-selective measurements and the jump equation (2) should be averaged over the outcomes, according to the probability distribution (4). The mathematical model of the GRW theory reduces to the following master equation for the density matrixρ ^: dρ^ i X Z = − [H;^ ρ^] + λ dx pG(x − x^ )^ρpG(x − x^ ) − λρ^ dt k k k k k ~ k i ^ X = − [H; ρ^] + λ D[x^k]^ρ. (5) ~ k The decoherence superoperator is defined by Z D[x^]^ρ = dxpG(x − x^)^ρpG(x − x^) − ρ.^ (6) We can analytically calculate it in coordinate representation ρ(x; x0) of the density matrix. Its contribution on the rhs of the master equation (5) shows spatial decoherence, saturating for large separations: 0 0 2 dρ(x; x ) X (xk − x ) = ::: − λ 1 − exp − k ρ(x; x0); (7) dt 8σ2 k where ellipsis stands for the Hamiltonian part. The amplification mechanism is best illustrated in c.o.m. dynamics. As we said, for the individual particles the decoherence term remains negligible whereas for bulk degrees of freedom, e.g.: the c.o.m., it becomes crucial to damp Schr¨odingercats, as we desired. Assume, for simplicity, free spatial motion of a many-body object. Then the non-selective 5 GRW equation (5) yields the following autonomous equation for the reduced c.o.m. density matrixρ ^cm: dρ^cm i ^ = − [Hcm; ρ^cm] + NλD[x^cm]^ρcm: (8) dt ~ As we see, the decoherence term concerning the c.o.m. coordinate has been amplified by the number N of the constituents [3] ensuring the desired fast decay of macroscopic superposi- tions: dρ (x ; x0 ) (x − x0 )2 cm cm cm = ::: − Nλ 1 − exp − cm cm ρ(x ; x0 ): (9) dt 8σ2 cm cm In the selective evolution the individual GRW measurements (2) entangle the c.o.m., rota- tion, and internal degrees of freedom, hence jΨcmi does not exist in general. It does in a limiting case of rigid many-body motion when the unitary evolution of jΨcmi is interrupted by spontaneous σ-precision measurements of the c.o.m. coordinatex ^cm similarly to (2) just the average rate of the measurements becomes Nλ [20] instead of λ. III. LOCALIZATION IS NOT TESTABLE, BUT DECOHERENCE IS Standard concept of selective measurement implies that we have access to the measure- ment outcomes, which are the values xk in GRW. If they are accessible variables then the stochastic jump process of the GRW state vector jΨi is testable otherwise it is not.
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